1. Field of the Invention
The present invention relates to the identification and location of fiber optic cables.
2. Summary of the Prior Art
There are many methods known for identification and location of underground objects. Many of these work by the detection of a magnetic field that is generated by electrical current carried by the underground object. These methods are effective for such objects as metal pipes, electrical cables and fiber optic cables which carry a metal sheath. The electric current may already be carried by the underground object, ie. in an electrical cable, or may be applied thereto, eg. by applying a voltage to the sheath of a fiber optic cable, or induced by applying a magnetic field to the underground object. However many fiber optic cables do not contain any conducting material, and therefore the known methods of location and identification of underground objects cannot be used.
The present invention provides a system for the identification and location of fiber optic cables which makes use of the Faraday Effect. In EP-A-0390341, a method and apparatus are described for identifying an optical fiber carrying a beam of polarised light. If a magnetic field is applied to the beam with a component axial to the direction of polarised light, the polarisation of the light is rotated by this axial component. EP-A-0390341 thus proposes applying a magnetic field with an axial component to the fiber, and detecting the change in polarisation of the light at a remote location. In this way, it can be confirmed that the cable in which the change in polarisation is detected is the same cable to which the magnetic field with an axial component is applied.
The magnetic field may be applied to the fiber optic cable in a number of ways. Firstly, it can be generated above ground, using coils or spinning magnets. This method has the disadvantage that the magnetic field may affect the polarisation of light carried in other fiber optic cables nearby.
Alternatively, coils may be placed around the cable and the signal from the signal generator applied directly to the coils. However, unless the coils are actually wound around the fiber, or effectively configured that way using a split coil with a connector strip each side of the fiber, the line integral of the net magnetic field component along the axis of the fiber will be zero or negligible. This effect is observed because the field at one point along the cable will be cancelled out by a reverse field at a second point along the cable. By Ampere""s law, the line integral of the magnetic field around an arbitrarily chosen path is proportional to the net electric current enclosed by the path. Thus, such use of an electromagnetic coil will not generate a detectable modulation of the polarisation of light.
The present invention therefore provides a method of effecting Faraday rotation of light travelling through a fiber optic cable. The method uses an arrangement in which an electromagnetic field is propagated towards the fiber, such that it traverses the fiber in an essentially transverse direction with a time-varying magnetic field component oriented along the length of the fiber, that component varying such that the line integral thereof along the cable is non-zero.
The light must have a degree of polarisation which is sufficiently great to be detectable. In principle, the light should be wholly polarised, but this is not always possible to achieve and a suitably high degree of polarisation is sufficient.
It is also possible, in principle, for the frequency of the electromagnetic field to have any value. However, the frequency affects the size of the antenna which is to generate it, and so radio frequencies, e.g. in the range 10 kHz to 200 MHz, preferably in the range 10 MHz to 200 MHz, may be used.
The field can be generated by any suitable radio frequency (RF) or microwave antenna, such as, for example, a simple dipole antenna. The propagating field causes the plane of polarisation of light passing along the cable to be modified, but without exhibiting problems due to the field reversal effect described above. Note that the size of the antenna needed increases with decrease in frequency, which is why radio frequencies are preferred.
Preferably, the antenna is aligned to extend in a plane which is parallel or substantially parallel to the longitudinal axis of the cable, but offset from it, with the axis of the or each dipole of the antenna oriented perpendicularly to the axis of the cable. Preferably, an array of antennas is used, which generates a strong beam directed towards, or focussed onto, the fiber optic cable.
Suitable arrays include a Yagi linear array, or an array of dipoles in which each dipole lies on part of a conceptual cylindrical surface encircling the fiber optic cable, with the axis of each dipole being aligned perpendicular to the direction of the fiber axis. Alternatively, a parabolic dish antenna can be used to form a collimated RF or microwave beam crossing the cable, with its magnetic field direction aligned with the direction of the cable.
Thus, at its simplest, the present invention may involve the input to the fiber of plane polarised light, the application of the electromagnetic field with a component with a non-zero line integral, and the detection in the resulting change of polarisation.
However, in some situations, this simple arrangement may not be wholly successful. As a plane polarised light beam propagates down a fiber optic cable, its state of polarisaton may vary. The state of polarisation is the relationship between the magnetic and electric components of the electromagnetic field of the light and these are affected by geometrical factors, and also the inherent birefringence of the fiber. It is possible for the state of polarisation to vary such that an initially plane polarised beam is, at some point along the fiber, circularly polarised. If the point of application of the electromagnetic field with a component with a non-zero line integral happens to be such a point where the light is circularly polarised, the effect of the present invention cannot be detected.
It is therefore important that the state of polarisation at the point of application of the electromagnetic field is not circularly polarised. However, this is not always possible to achieve, since there are many factors which affect the state of polarisation along the fiber.
Therefore, a development of the present invention proposes to overcome this problem by applying a modulation to the state of polarisation, which modulation changes with time. The modulation causes the state of polarisation to vary at the point of modulation, and hence at all subsequent positions along the fiber. Therefore, even if the point application of the electromagnetic field happened to coincide with a circularly polarised state of polarisation at some time, the modulation would ensure that, at some later time, the beam was not circularly polarised since its state of polarisation would have changed due to the modulation.
The modulation could be wholly random, or could be regular as desired. Whichever is chosen, the effect is that the electromagnetic field is applied to the non-circularly circularly polarised state at least for part of the modulation cycle.
Means for achieving polarisation modulation are know, and may include mechanically perturbing the fiber in a cyclable or randomly-varying manner, by applying an axial or bending strain, or using a electro-optic or magneto-optic modulator with a time-varying electrical drive signal.
The modulation applied to change the state of polarisation should be sufficiently different from the frequency of the electromagnetic field which is applied to be separable by suitable signal processing. It is possible for the modulation to have a similar frequency to the electromagnetic field which is applied to the fiber, but for signal processing reasons it is preferable for them to be different. Thus, the polarisation modulator may typically operate at frequencies from a few Hz to a few kHz, but if the electromagnetic field has a frequency in the range 10 MHz to 200 MHz, it would be possible for the modulation frequency to be as high as a 100 kHz and still easily be separable by signal processing.